Reliable Detection of Rotor Faults Under the Influence of Low-Frequency Load Torque Oscillations for Applications With Speed Reduction Couplings

Low-frequency torque oscillations in the load can induce frequency components in the vicinity of that of rotor faults (RFs) resulting in false alarms when applying motor current signature analysis (MCSA). False RF indications due to load oscillations (LO) are most common in applications that employ speed reduction couplings for high torque, low-speed operation. Recently, ideas for separating RF and LO have been proposed in the literature; however, the case where two components overlap at the same frequency has not been investigated. Several cases where RF- and LO-induced components are identical have been observed in the field by the authors with commercial MCSA equipment. It is shown in this paper that overlap between the two components can produce a false positive or false negative indication because they can add or cancel depending on the relative phase between the components. Alternative options for reliable RF testing among existing test methods are evaluated and verified in this paper for cases where the two components overlap and produce false indications.

I. Introduction

Condition monitoring of industrial induction motors based on the current spectrum, known as motor current signature analysis (MCSA), has become common in the field for induction motor testing. Its remote monitoring capability from the motor control center using existing current transformers makes it an attractive and convenient tool to apply in an industrial environment for medium-high-voltage motors. Numerous case studies reported in the literature show that MCSA is capable of detecting rotor bar and end ring breakages for preventing potential forced outages due to rotor failure [1]–[8]. However, false fault alarms (false positive indication) or missed faults (false negative indication) are common in the field as reported in [4]–[9].